Birch rule

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Baeger-Villiger reaction, 311 Barbiturates, 358 Base peak in ms. 248 Bases, soft and hard. 121 Basicity and structure. 43 Basic Red. 428 Beckmann rearrangement. 406 Benzene, resonance structure. 192 structure, 189 Benzenonium ion, 205 Benzhydrol, 269 Benzidine rearrangement. 422 Benzyne. 217 Bicydic compounds. 162 Birch reduction, 200 Bischler-Napieralski reaction, 460 Boat and chair forms. 168 Boiling point, influences on, 212 Bond dissociation, 37 Bond order, 17 Bond stretching. 233 Bonding orbitals, 14 Bredt s rule. 168 Bromonium ion. 100 BrOnsted. 42... 

Methoxybenzene (anisole) is reduced more rapidly than benzene under Birch conditions. This is an exception to tiie rule, the reasons for which are not clear,... 

Solutions of Group I metals in the lower molecular weight amines are more potent reductants than those in liquid ammonia, and as a general rule substrates are more extensively reduced than by the Birch method. o Naphthalene (49 Scheme 48), for example, is reduced by a solution of lithium in ethylamine to a 1 1 mixture of A W- and A -octalins (214) and (215). If ethylenediamine is employed as the medium, the completely saturated decahydronaphthalene is formed, while the proportion of (215) may be increased to 80% by utilizing a (1 1) mixture of ethylamine with dimethylamine. The formation of the more-substituted alkene appears to be a general result for such primary and secondary amine mixtures and has been used to good effect in the reduction of both toluene and cumene to their 3,4,5,6-tetrahydro derivatives (216) and (217), respectively, in ca. 80% yields. A comprehensive review of these kinds of reducing systems, which also draws comparisons with the Birch method, is available,but more recent-... 

The optically active compound can be prepared in the same way as mentioned above. We started with the ketone 58, which was treated under Birch reduction conditions followed by NaBH4 to afford a P-alcohol 97. The camphanoyl derivative of this alcohol was separated by HPLC to give 98 and 99. The ketone (+)-100 derived from 98 showed a negative Cotton effect in the CD spectrum. The absolute configuration of this ketone must be formulated as depicted for (+)-100 from the Octant rule. The ketone (+)-100 was converted to (-)-84 in five steps as used in the former synthesis. The specific rotation of (-)-84 is [a]D -11.5 (c=4.2, CHClj) (lit. [a]D -3.0° (c=0.17, CHCI3) (20)). Therefore the absolute configuration of the natural product is formulated as (-)-84. 

Nortestosterone and 19-norprogesterone were considerably easier to obtain as soon as Arthur John Birch (1915-1995) had discovered the reduction of aromatics. [36] While the reactivity of deprotonated oestrone is too low for a direct Birch reduction, the methyl ethers were used in particular, because electron donors lower the reaction rate. The donor substituent is always attached at the remaining double bond, and the product is, on account of the principle of least motion, a 1,4-cyclohexadiene, in this case the 2,A -doubly unsaturated steroid skeleton. The second conceivable isomer, with double bonds at the 3,6-positions, is for steric reasons thermodynamically less favoured (Bredt s Rule). [37]... 

Birch reduction 844 heterocycles 833 Huckel s rule 827 meta 820... 

Reduction with Alkali Metals, Various methods of reduction using alkali metals have come into use in total steroid synthesis in liquid ammonia, in ammonia with the addition of alcohols (Birch reduction), and in alcohols. The working procedures and mechanisms of these reactions have been discussed in appropriate reviews [87-89] in general, they take place through the formation of anions or anion-radicals with their subsequent protonation. As a rule, in these reactions isolated double bonds are not reduced the sequence of reduction of other unsaturated groupings is as follows a, -unsaturated ketones > slyrene or stilbene double bonds > aromatic rings. Each of these cases has its own stereochemical problems, which are discussed below on the basis of suitable examples. 

The validity of the propionate rule of Birch et al. (i960) for methymycin synthesis was demonstrated by assuming that the Cg unit, represented by the carbon atoms C-7 and C-8, was derived from an acetate precursor. An alternate suggestion by Bentley (I962) was that even the Cg unit might have come from a propionate unit whose methyl group was oxidized and subsequently decarboxy-... 

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